1. Field of the Invention
The present invention relates to receiver apparatuses, receiving methods, programs, and recording media. More particularly, the present invention relates to a receiver apparatus, a receiving method, a program, and a recording medium, which are capable of improving the reception characteristics.
2. Description of the Related Art
Hitherto, fractional sampling or over-sampling in which a signal corresponding to one symbol is sampled multiple times are proposed for wireless receiver apparatuses.
For example, tradeoff between sampling intervals and characteristics in Direct Sequence Spread Spectrum (DS/SS) modulation methods (for example, refer to K. J. Kim, S. Y. Kwon, E. K. Hong, and K. C. Whang, June 2000, “Effect of Tap Spacing on the Performance of Direct-Sequence Spread-Spectrum RAKE Receiver”, IEEE Trans. on Commun., Vol. 48, No. 6) and effects of diversity by the fractional sampling in Orthogonal Frequency Division Multiplexing (OFDM) modulation methods (for example, refer to C. Tepedelenlioglu and R. Challagulla, November 2004, “Low Complexity Multipath Diversity through Fractional Sampling in OFDM”, IEEE Trans. on Signal Processing, Vol. 52, No. 11) have been already discussed. The discussion has showed that an increase in the sampling rate improves the characteristics.
Mechanisms of estimating channel responses after oversampling reception signals to detect appropriate despread timing have been proposed for receiver apparatuses in Code Division Multiple Access (CDMA) systems (for example, refer to Tu Chunjiang, Zhou Xin, Liu Bo, and Chen Hongyi, October 2003, “The design of 802.11b WLAN baseband processor”, Proceedings. 5th International Conference of ASIC, 2003, Vol. 2, 852-855).
Such a mechanism is exemplified by a method using sliding correlation (for example, refer to US Patent Publication No. US2003/0123408 “CDMA Receiving Apparatus”; US Patent Publication No. US2004/0139466 “Finger Allocation for a Path Searcher in A Multipath Receiver”; and J. Mitsugi, M. Mukai, and H. Tsurumi, September 2002, “Path-search algorithm introducing path-management tables for a DS-CDMA mobile terminal”, Proceedings. The 13th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Vol. 2, 730-734). FIG. 1 is a block diagram showing an example of the configuration of a receiver apparatus 1 receiving signals by a channel response estimation method by using the sliding correlation.
Referring to FIG. 1, a received signal is input in a sliding correlator 11. The sliding correlator 11 multiplies the received signal by a spread code having a phase T, supplied from a spread code generator 12. The signal resulting from the multiplication is supplied to an integrator 13. The integrator 13 integrates the signal with respect to the period of one spread code, and the integrated signal is supplied to a square circuit 14. The square circuit 14 detects a channel response corresponding to the phase T. The output from the square circuit 14 is supplied to a channel response estimator 15. The channel response estimator 15 estimates the entire channel response corresponding to different phases other than the phase T.
The above mechanism is exemplified by another method using correlation (for example, refer to H. Hamada, M. Nakamura, T. Kubo, M. Minowa, and Y. Oishi, May 1999, “Performance evaluation of the path search process for the W-CDMA system”, Proceedings. 1999 IEEE 49th Vehicular Technology Conference, Vol. 2, 980-984 and US Patent Publication No. US2004/0013218 “Receiving Device and Receiving Method”). FIG. 2 is a block diagram showing an example of the configuration of a receiver apparatus 21 estimating a channel response by using the correlation.
Referring to FIG. 2, a received signal subjected to oversampling in an analog-to-digital (A/D) converter (not shown) is input in a decimator 31 where the signal is downsampled at appropriate sampling intervals. The output from the decimator 31 is supplied to a correlation filter 32 where correlation processing (matched filtering) is performed. If the received signal has predetermined correlation, the output from the correlation filter 32 directly represents a channel response. Accordingly, a channel response estimator 33 estimates the entire channel response on the basis of the output from the correlation filter 32. It is possible to estimate the channel response in a short time by this method.
The mechanism is further exemplified by a method using multiple correlators (for example, refer to U.S. Pat. No. 6,487,193 “Path Searched Device and CDMA Receiver with the Same”). FIG. 3 is a block diagram showing an example of the configuration of a receiver apparatus 41 capable of using two correlation filters to estimate individual channel responses only if necessary.
Referring to FIG. 3, a received signal is input in a decimator 51. The decimator 51 decimates the received signal at two decimation rates. The decimator 51 supplies a decimation output at a higher decimation rate (a lower degree of oversampling) to a correlation filter 52 and supplies a decimation output at a lower decimation rate (a higher degree of oversampling) to a correlation filter 53. The correlation filter 52 roughly estimates channel responses, and the operation of the correlation filter 53 is delayed for a necessary time by using the estimated result by the correlation filter 52 to estimate the channel responses in detail. A channel response estimator 54 estimates the entire channel response on the basis of the channel responses estimated by the correlation filter 53.
The technologies described above with reference to FIGS. 1 to 3 each detect the maximum value of the impulse response in one chip and does not detect two or more samples per chip.
In recent years, communication technologies using multiple antennas, such as Multiple Input Multiple Output (MIMO), Single Input Multiple Output (SIMO), and Multiple Input Single Output (MISO), have received widespread attention. For example, in the MIMO, both a transmitter apparatus and a receiver apparatus are provided with multiple antennas or multiple antenna elements (elements each including at least one antenna) to realize multiple transmission lines that are logically independent of each other. The receiver apparatus splits a received signal and simultaneously demodulates the signal to achieve both improvement in the frequency usage efficiency and reduction in the error rate owing to the effects of diversity (for example, refer to Motohiko Isaka, December 2003, “MIMO Tsushinro ni Okeru Fugouka to Hennchou (Coding and Modulation in MIMO Channel)”, The Institute of Electronics, Information, and Communication Engineers (IEICE) Transactions A, Vol. J86-A, No. 12 1292-1302).